This invention relates to molded case circuit breakers operable upon high fault currents to limit the peak let-through current.
Circuit breakers of this type are generally known which employ separable contacts supported in a manner to direct the current flow through the contact support members in opposite directions whereby electromagnetic forces generated by these currents operate to separate the contacts independently of the circuit breaker operating mechanism. The contact structure is designed to quickly separate the contacts during a high fault current condition to rapidly lengthen the arc generated between the contacts, thereby to rapidly raise the arc voltage. Magnetic devices are utilized to amplify the flux and the electromagnetic forces generated by the currents in the contact support members to thereby enhance the separating movement of the contacts. These magnetic amplifiers are generally U-shaped structures of magnetizable material disposed around the supporting elements for the separable contacts.
A problem associated with molded case circuit breaker design is the ability to package a contact structure having a desired current interruption capacity into a molded case having specific dimensional constraints. For example, 600 volt industrial frame molded case circuit breakers of the aforementioned type are known which have one or more pairs of cooperating separable contacts each pivotally mounted for movement in opposite directions to increase the relative separation rate therebetween. While this approach is satisfactory for industrial frame breakers, the vertical space requirements for a contact structure of this type render it unsatisfactory for use in 240/250 volt residential and commercial frame molded case circuit breakers which are intended to mount within flush mounted panelboards located in dwelling studwalls.
Another problem associated with high current interrupting molded case circuit breakers is the ability of the molded case to withstand the shock forces associated with arresting the movement of the contacts under high fault current separation and the forces exerted on sidewalls of the case by internal pressure developed by gasses as a result of an arc established during contact separation. The invention disclosed herein relates to improvements in molded case circuit breakers which particularly address the aforementioned and other problems.